Sabermetrics is the analysis of baseball through objective evidence, especially baseball statistics. The term is derived from the acronym SABR, which stands for the Society for American Baseball Research. It was coined by Bill James, who has been its most enthusiastic (and by far its most famous) proponent.

From David Grabiner's Sabermetric Manifesto:

Bill James defined sabermetrics as "the search for objective knowledge about baseball." Thus, sabermetrics attempts to answer objective questions about baseball, such as "which player on the Red Sox contributed the most to the team's offense?" or "How many home runs will Ken Griffey, Jr. hit next year?" It cannot deal with the subjective judgments which are also important to the game, such as "Who is your favorite player?" or "That was a great game."

Sabermetricians call into question traditional measures of baseball skill. For instance, batting average is considered to be a statistic of limited usefulness because it turns out to be a poor predictor of a team's ability to score runs. Typical sabermetric reasoning would say that runs win ballgames, and so a good measure of a player's worth is his/her ability to help his/her team score more runs than the opposing team.

Accordingly, sabermetric measures - such as Bill James's Runs created and Win shares or Pete Palmer's Total player rating - are usually phrased in terms of either runs or team wins; a player might be described as being worth 54 runs more than an average player at the same position over the course of a full season, for example.

Sabermetrics is concerned both with determining the value of a player in a season gone by, and with trying to predict the value of a player in the future based on his past performances. These are not the same thing. For example, a player with a high batting average one year may have been very valuable to his team, but batting average is known to be a volatile stat and relying on it to remain high in future years is often not a good principle. A sabermetrician might argue that a high walk rate is a better indication that a player will retain his value in the future.

While this area of study is still in development, it has yielded many interesting insights into the game of baseball, and in the area of performance measurement generally.